Customer or user data are recorded on optical disks as marks written at high lineal densities into records within sectors in the optical disks. While customer data are recorded at a given lineal density, the frequency of marks recorded on optical disks varies in accordance with the (d,k) constraints of the run length limited (RLL) codes. Some format or control fields of the sectors specified in known optical disk standards are written into with a given high mark density pattern. One of these fields is the flag field called out by the American National Standards Institute (ANSI) standard for unbanded write-once read-many (WORM) media. Once recorded, a high mark density field, such as a flag field, is read back into a pulse stream which has a given high frequency of pulses corresponding to the given high mark density. A purpose of the flag field is to provide a control for preventing overwriting user data. In particular, this invention deals with high reliability signal detection techniques for detecting a relatively short high mark density pattern. This invention enables faithfully detecting each flag field to allow protection of customer data from inadvertent overwrite.
The American National Standards Institute (ANSI) standard for unbanded WORM optical disk define this flag field in each sector on the disk. The sector topology of this standard was broadened by IBM. Such broadened topology is incorporated by IBM into formats for radially-banded WORM media. Each flag field is disposed between a sector's embossed identification (ID) field and the customer/user data field. The flag field is written into only when data has been written into the customer data field of the sector. In write-once media, such flag field recording indicates that the sector has been written to and is now a `read only` sector. That is, writing to a previously written sector is prohibited to prevent obliterating the previously written data. Also, in ablative WORM media that is grooved to indicate track location, such over-writing of written data may totally obliterate the raised track (land) between the grooves into which the marks are recorded. This additional ablation makes such groove optically indistinguishable from the adjacent grooves, thereby degrading track following over this sector. Such over ablation also degrades track seeking in which a laser beam happens to traverse this over-ablated sector.
Writing data to a WORM optical disk may require two disk rotations. A first disk rotation reads the flag field for verifying that no high mark density pattern has been written in this format field and then proceeds to write the customer data field if the flag field is blank, indicating an unwritten sector. A second disk rotation then writes the high mark density pattern in the flag field followed by a verification the customer data field has been written satisfactorily. The sector ID is usually embossed in a leading portion of the sector using the same high lineal density RLL coding as the written data is recorded at. For ensuring that no written data is inadvertently overwritten, detection of the flag field must be robust and accurate.
Further, WORM devices typically scan the WORM disk for finding a first unwritten sector, this operation being referred to as a medium scan operation. That is, WORM disks are typically partitioned in such a way that sectors will be written to sequentially by an application such that a first unallocated sector is the first of many unwritten sectors. This so-called medium scan uses flag field indications, in addition to checks performed on the customer data fields, to determine whether a sector has been written to. Therefore, it is important to robustly read each flag field to ensure there is no signal misdetection leading to an incorrect indication of a first unwritten sector. This error can occur if either a written flag field is missed or a blank flag field is falsely detected as being written. In the former case, a sector which has a flag field protecting a customer data field that is marginal (e.g. obstructed by a media defect or written at a very low power) and which may not be detected by the media scan can be sensed as being blank (unrecorded). In the latter case, false detection of a flag field which is actually blank and which precedes an unused customer data field (i.e. the sector is blank) can cause the medium scan operation to decide that a second blank sector is the first sector available for use, causing a blank sector to be left among the written sectors. If a subsequent medium scan operation correctly determines this sector is blank, a multiple sector write may be initiated starting at this sector. The attempted write of the second sector of this multiple sector operation is actually attempt to overwrite the previously written sector. If this write is not aborted by proper detection of the flag field, customer data will be overwritten and lost.
Embossed sector ID's may be faulty resulting in unreliable read back from the disk. That is, one device could read a given sector ID while another device fails to read the same sector ID. Also, data for storage in sectors may be reassigned because of some marginal operating condition, i.e. a faulty ID or a media defect. Media defects may appear as signals, particularly in ablative WORM media. Further, diverse manufacturers may have diverse criteria for qualifying sectors before writing to a sector. In any event, accurately and robustly detecting whether the flag field has been recorded measurably improves medium scanning reliability and is necessary for reliable WORM data recording operations.
To prevent unintended data overwriting in a WORM medium, read back signal processing of the flag field must robustly detect whether the high mark density pattern has been recorded therein. The read back circuits distinguish between real signals and noise induced by scanning unwritten areas of the disk. Typically, read back signal processing for sector ID's immediately preceding each flag field requires detection of all read back signals of embossed indicia for ensuring accurately accessing addressed sectors. Reliable detection of sector IDs and customer data field at high lineal densities, such as used in optical recording, requires read equalization to boost the higher frequency readback signal components. Read equalization reduces the dynamic range of the signal (ratio of high to low frequency signal amplitudes) and cancel inter-symbol interference (ISI). Read equalization which boosts high frequencies tends to shape media noise "readback" when scanning an unwritten sector in such a way that an unqualified peak detector would generate a high frequency pulse stream easily confused with the readback pulse stream resulting from a high mark density pattern such as is written to the flag field. Therefore, a later described qualified-peak detector should be used in data read channels.
Further, for reliably reading high lineal density unipolar recorded signals a so-called inverted read channel is used for enhancing reliability of reading such written high-lineal-density signals. Such an inverted read channel operation simplifies peak following and qualification threshold generation. The readback signal peak qualification allows the read detector to reliably sense high mark density patterns within run-length-limited (RLL) encoded data that is not ideally written resulting in undesired low peak-to-peak amplitude readback. Such a channel rejects the DC component of the readback in favor of sensing signal peaks of the read back signal. Therefore, such an inverted read channel more reliably reads the high lineal density signals written in accordance with the (d,k) constraints of an RLL code than a usual non-inverted qualified peak detector. Such an inverted read channel does not reliably distinguish between the high mark density pattern written into a flag field and media noise shaped by read equalization. Therefore, for acceptable operation on WORM media, the read detection of the optical disk player must provide robust detection of both random data recorded at high lineal densities in accordance with known (d,k) RLL code and control information written into the flag field at a given high mark rate or pulse repetitive frequency.